This invention relates to an automatic transmission, particularly to a line pressure control device provided with a hydraulic control circuit having cutback means.
Generally, an automatic transmission includes a torque converter, a transmission gear mechanism having a planetary gear unit and friction coupling members for shifting the gear ratio, and a hydraulic control circuit for controlling hydraulic pressure to the respective friction coupling members. The friction coupling members are clutches, brakes, and the like. The hydraulic control circuit includes pressure regulator means for regulating the hydraulic oil supplied from an oil pump to an appropriate line pressure, and shift valves for controlling the feeding and releasing of coupling pressure to the respective friction coupling members. In such an automatic transmission, the line pressure has been required to control in accordance with change of transmitted torque in order to develop a coupling pressure suitable to transmit the torque, and assure a reduced driving loss of the oil pump and the like.
In view thereof, there has been proposed an automatic transmission provided with a hydraulic control circuit having cutback means. In such an automatic transmission, in addition to usual control of controlling the line pressure in accordance with the throttle opening of the engine, the so-called cutback control is executed in a particular gear ratio. In the cutback control, specifically, the line pressure is reduced when the transmission is set in the particular gear ratio. For example, Japanese Examined Patent Publication No. 46-33050 discloses an automatic transmission having a control valve responsive to hydraulic pressures in shifted gear ratios for reducing the line pressure stepwise as the transmission is progressively set in higher speed gear ratios, in other words, the control valve is adopted for executing cutback control.
In the above-mentioned automatic transmission, when the transmission is set in a low speed gear ratio to transmit a great torque, the cutback control is not executed to maintain a high line pressure to develop a necessary coupling pressure. When the transmission is set in a high speed gear ratio to transmit a small torque, the cutback control is executed to give a reduced driving loss to the oil pump.
There has generally been known an automatic transmission having a coast clutch in its transmission mechanism which enables engine brake. Similarly to other friction coupling members, the coast clutch is locked and released by a hydraulic control circuit. In respect of a particular gear ratio, e.g., second gear ratio, the coast clutch is released in a usual running range, e.g., D range and is locked in an engine brake enable range, e.g., 2 and 1 ranges.
In the automatic transmission in which even in the same gear ratio, the coast clutch can be locked or released in accordance with a selected range, friction coupling members are to slip when the vehicle is decelerated by carrying out the engine brake in the particular gear ratio in the engine brake enable range. Accordingly, it will be seen that the cutback control is necessary to be suspended not only when in low speed gear ratios but also when in the particular gear ratio in the engine brake range.
To fulfill the above-mentioned necessity, that is, to control the execution of cutback not only in accordance with gear ratios but also in accordance with ranges, conventional automatic transmissions are provided with very complicated hydraulic control circuits.